High resolution profiler piggy-backing on top of gcc -finstrument-function.

Introduce a new -finst option which allows to hook into all the functions which
were instrumented by gcc via the -finstrument-function option.

A few remarks:

* For the moment it only works if the code being instrumented is single threaded.
* Because of limitations of -finstrument-function, it is not possible to instrument
  code which uses SSE intrinsics.
Avoid using a signed variable when doing shifts.
Do not use `times` as a global variable to avoid conflict with the `times` function.
Merge branch 'master' of https://github.com/ktf/igprof

Conflicts:
	src/profile-finstrument.cc